Avoid tox_install.sh for constraints support

We do not need tox_install.sh, pip can handle constraints itself
and install the project correctly. Thus update tox.ini and remove
the now obsolete tools/tox_install.sh file.

This follows https://review.openstack.org/#/c/508061 to remove
tools/tox_install.sh.

Change-Id: I42b7dcd3e9e41688ac3bd8b9c9fb9b39f90c3e9a
diff --git a/tox.ini b/tox.ini
index 196d873..3163c7d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -5,13 +5,13 @@
 
 [testenv]
 usedevelop = True
-install_command = {toxinidir}/tools/tox_install.sh {env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
-deps = -r{toxinidir}/requirements.txt
+install_command = pip install {opts} {packages}
+deps =
+       -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+       -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 setenv =
     VIRTUAL_ENV={envdir}
-    BRANCH_NAME=master
-    CLIENT_NAME=designate-tempest-plugin
   PYTHONDONTWRITEBYTECODE=1
 whitelist_externals = sh
                       find